This invention relates generally to X-ray generators and, more particularly, to a method and apparatus for providing a variable dead time in a pulse-width-modulated inverter adapted to supply power to an X-ray tube.
The use of an inverter in an X-ray generator circuit to provide an alternating current to the primary high voltage transformer is known. However, due primarily to the fact that they are relatively difficult to control, transistors have generally not been used for this purpose. Rather, it has been the thyristor which has been used for the switching device in these applications. Although thyristors are considered to be generally rugged and relatively easy to control, they have the inherent disadvantage of requiring the use of forced commutating circuitry. Thus, not only is there a need for extra components, but also, the added circuitry tends to limit the maximum system operating frequency and to also substantially increase the circuitry losses.
The use of transistors as a switching device in an inverter is commonly made in many applications. One such application is in the transportation field where there is a requirement for controlling high power at medium voltage levels. However, in the X-ray generator field, where there is a requirement for not only high voltages involved but also a desire to operate at higher frequencies to minimize the ripple on the output waveform, the use of transistors as switching devices in inverters has not been made. One of the reasons is that a transistor is generally less rugged than other switching devices, such as the thyristor. Another reason for not using transistors in such an application is that, until recently, there have been few, if any, transistors which are more capable of reliably operating at the required voltage and current levels (e.g., 600 volts, 300 amps.). Finally, it is generally understood that the use of transistors presents unique and difficult control problems that do not exist for other switching devices.
In conventional X-ray generator systems, the power level to the X-ray tube is varied by way of a variable input/output transformer whose primary windings are selectively closed by way of static contactors, such as, for example, thyristors. Such systems have various disadvantages as discussed in U.S. patent application Ser. No. 564,538, filed concurrently herewith and incorporated herein by way of reference. Although the concept of using pulse-width modulation to control the output of a transistor inverter has been used for certain applications, it has not heretofore been used in the X-ray generator field. This is partly due to the inherent disadvantages of transistors as discussed hereinabove. In addition, there are other specific problems which arise when a transistor inverter is applied to an X-ray generator.
In X-ray generators, it is desirable that the relatively high output voltage be controlled very accurately and have minimum ripple. Further, the system should have a fast response on the rise time to guarantee reproducible, short exposure times. It is recognized that these characteristics can be enhanced by operating at higher frequencies. However, such high-frequency operation is limited by the performance capabilities of the available components. For example, in a transistor inverter, one of the problems is that of minimizing the dead times on the transistor switching which in turn depend on the operating characteristics of the transistor itself, i.e., the associated storage and fall times thereof. Commercially available transistors handling currents of 500 amps., with collector-emitter open voltage capability of 1000 volts have storage and fall times which vary and are in the range of 6-35 microseconds (typically 20 seconds), depending on the manufacturer.
In order to minimize the output filter (improve the response) and the size of the high-tension transformer, it is desirable to operate at a relatively high frequency, i.e., in the range of 5-15 kHz. At these higher frequencies, a long or normal transistor storage time can seriously affect the control problem. For example, at 10 kHz, a total inverter period is only 100 microseconds and half a period is 50 microseconds. Accordingly, an uncontrolled storage time in the range of the typical 20 microseconds is very significant and will cause very serious distortion of the output voltage waveform.
Another limitation of transistors used in an inverter is the inherent sensitivity to excessive current. Since it is generally desirable to control the current flow in an inverter which has a critical current limitation, most inverter systems are provided with some kind of current limit protection. However, it is recognized that such protection networks have certain limitations. For example, the time required for a current limit circuit to complete its operation may be excessive so as to allow a second breakdown in the output transistors. This is so because, first of all, it is necessary to detect the excessive current on the output circuit. Secondly, it is usually necessary, for noise minimization purposes, to integrate the signal. And finally, it has to be compared with a reference value and, if it exceeds the threshold, it then has to be applied to cause the transistor controller to shut down the system.
Another problem which arises with square-wave inverters is one which is of particular concern with pulse-width-modulated systems. When a pulse-width-modulated transistor turns off, its series-connected transistor becomes reverse biased during the time period when its associated flywheel diode is conducting. If the pulse-width-modulated transistor is turned on during that period, then a large current spike can occur to cause damage to the devices.
A more serious result can occur from a phenomenon known as shoot-thru in an inverter. Such a condition is a short circuit across a pair of series-connected transistors in an inverter. This will occur if both of the series-connected transistors are turned on at the same time, which may occur in the event of a controls failure or because of noise that may be introduced to the system.
In order to provide a reliable output, an inverter system must accommodate the inherent variations that occur in the source and in the load. Moreover, there are also certain, occasional, unplanned conditions, such as an arc in the tube, which may occur on the high-voltage side which, if not controlled, may damage the inverter or the other components. Further, in any control network, there is a possibility of malfunction failure in the low-voltage control circuitry which, if not detected and attended to, may cause undesirable consequences within the control network itself or at the output of the system. Thus, with any control or performance-enhancing features that may be added to a conventional system, there are related monitoring and regulating features which must be provided to accommodate those enhancements.